Development of a Stream Rating Curve

Transcription

1 Development of a Stream Rating Curve Luis F. Andino Galeano landino@illinois.edu Abstract. Understanding the characteristics of streams is necessary to design and apply methods to achieve a sustainable management. Streamflow, Roughness Coefficient, and Rating Curve are elements in hydrology that help to a better understanding of this complex science. This study was conducted to observe and quantify those elements. Description of one crosssection, calculation of flowrate and Roughness Coefficient and the Manning s Equation were used to define physical properties of the stream in two sections. Both sites had similar conditions in the parameters previously mentioned. The calculations obtained from applying the Manning s Equation and Roughness Coefficient are a representative estimate of the observations in the field. Keywords. Roughness coefficient, Manning s equation, streamflow, and stream channel.

2 Introduction In East-Central Illinois, many agricultural fields require drainage systems to transport water to ditches and rivers. Understanding the characteristics of streams is necessary to design and apply methods to achieve a sustainable management. There are multiple aspects that can be studied in this field: flow, water quality, the composition of the bank and stream bed, biodiversity, ecological processes, chemical-physical properties, response to climate variables, and others. This experiment is focused on some of those elements and in the interaction that they could have. Certain activities or natural phenomena could have some effects on streams. In order to address those effects, it is necessary to gain knowledge and to have a clear interpretation of the characteristics involved in this area. This report summarizes field observations, calculations, and explanations about a particular stream in the Champaign County, Illinois. The overall objective of this report is to better understand basic concepts of open channels. The specific objectives are to determine a Roughness Coefficient from observation of field conditions, to develop a rating curve for a section of a section of an open channel, to develop a routine to solve for flow in natural channels and to develop a routine for solving Manning's Equation. Methods and materials Location. The study was conducted south of the Urbana/Champaign area, Champaign County, Illinois (Figure 1). The geographic coordinates in the Universal Transverse Mercator System of the segment of the observed ditch are Easting, Northing. The evaluated ditch drains mainly water from corn (Zea mays L.) and soybean (Glycine max (l.) Merr.) grown for research and educational purposes in its surroundings. Figure 1. Satellite image of the site of study (Google Earth, 2016)

3 Materials. Engineer s level, surveying rod, measuring tape and flags were used for field measurements. Data analyses and mathematical procedures were calculated in Microsoft Excel, and Google Earth provided imagery for illustrations and spatial observation. Methods. Measurements were taken on August 31 st, 2016 at the described location. The results presented in this report correspond to Group 2 of the Drainage and Water Management Course (ABE 459). Two additional groups performed similar measurements 147.5ft downstream (Group 1) and 53 upstream (Group 3). A cross-section of the open channel was divided into different sections (from a distance of 0, 6, 11, 13, 20, 24, 28, 32, and36 ft.) from one side to the other. The depth of the bottom of the channel and water surface level were assessed with a surveying rod and an engineer's level (with a common datum for all measurements on each point of the cross-section). This information was used to determine the channel shape at that cross-section. Additionally, stream flow rate was calculated using a floating object that traversed a known length at five points of the cross-section (4,5,8,11, and 14 ft from side to side) and timing three times in each section. The roughness coefficient (n) of the bank and stream bed were calculated using the Natural Stream Uniform Flow Spreadsheet (USDA 1993). The n equation (Limerinos, 1970) considers the hydrological radius (R) in feet which is the cross-sectional area divided by the wetted perimeter, and depth in feet (Limerinos, 1970) R 1 6 n = log ( R d ) Flow rate (Q) was calculated using the Manning Equation, where A is the cross-sectional area, R is equal to A divided by the wetted perimeter, n is the Roughness Coefficient and is constant to change the result to Imperial System units. For this report, it was assumed that the Manning Equation was always applicable. Q = A R2 3s 1 2 n Routines to estimate flow (Q) were development based on the n equation (Limerinos, 1970) and the Manning equations. The first routine contains data from two cross sections and the second one for solving Manning Equation in rectangular and trapezoidal channels.

4 elevation (feet) Results and discussion There is a difference of 5.97 ft. between the bottom of the open channel and the highest point contained in the cross-section (Table 1). Although the cross-section of the channel had a similar shape along its path, there could be some changes. The deepest point of the stream is located close to the third measured station, not in the middle (Figure 2), this could be an indicator of higher erosion rates due to steep slopes. Table 1. Calculation of the elevation of the channel cross-section Tape(ft) HI Rod (ft) Rot (in) Rod Elevation1 Distance Diff. elevation from bottom station (feet) Figure 2. Stream cross-section shape. The roughness coefficient (n) of the bank and stream bed are 0.04 and respectively. The stream bank is composed mainly of Earth, with moderate irregular sides, with gradual changes in size and shape of cross section, no obstruction and a high retardant effect of vegetation. The stream bed contained fine gravel, has smooth irregularities on the sides with occasional

5 ftᵌ/s n changes in size and shape of the cross-section and no effect of obstruction and retardance due to vegetation. This n value of the site evaluated by group 3 indicates that the roughness coefficient increases when the depth is higher. The n value of the site evaluated by group 2 is more constant n G2 n G Depth (ft) Figure 3. Manning s n value for depth. The rating curve indicates that depth increments will increase flow at higher rates, especially to the section measure by group 2 (Figure 4). Although the sites analyzed by groups 2 and 3 should have a similar flow, physical differences in the cross-section will prevent them from being equal. Tile outlets could also make differences in flow between two close sites, which are not easily distinguishable in some cases Q G2 Q G Depth (ft) Figure 4. Rating curve of sites analyzed by groups 2 and 3

6 The routine to calculate Q for the two cross-sections (2 and 3) indicate that different values for the analyzed characteristics may result in equal estimation of streamflow (Q) (Table 2). A bed slope of 0.01 was considered between sites. Routine between points 1 and 3 were not possible to calculate due to some measurement errors in site 1. Routine to calculate Manning s equation is in the attached Excel file. Table 2. Result of the routine to calculate Q in two cross-sections Bed Slope: 0.01 A R n n-prime Q Grupo Grupo Conclusions There are similarities between the physical characteristics of the sites of the stream analyzed by Groups 2 and 3. The distance between them did not influence flow, Roughness coefficient, and shape of the cross-sections. The Manning s Equation is applicable to the stream where this study was conducted because consistent results were obtained through that method. The calculations are a representative estimate of the observations in the field. Besides applying adequate methods to obtain information related to roughness, cross-sectional areas or flow rates, it is important to properly use the tools, techniques to avoid unwanted variations in the results. Precision is desired in every study. Likewise, the proper interpretation of the data leads to better decisions in the management of the streams. References Google Earth. (September, 14, 2016). Champaign County, Illinois. UTM 16N E N Limerinos, J. T. (1970). Determination of the Manning coefficient from measured bed roughness in natural channels. USGS water Supply Paper 1898-B: 52 pages. USDA Natural Stream Uniform Flow Spreadsheet. Natural Resources Conservation Service. Available at: Accessed September 14, 2016.